SC183C

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13

Detailed Description

The SC183C is a synchronous step-down converter utiliz-

ing a 2.5MHz fixed frequency voltage mode architecture.

The switching frequency is chosen to minimize the size of

the external inductor and capacitors while maintaining

high efficiency.

Operation

During normal operation, the PMOS MOSFET is activated

on each rising edge of the internal oscillator. The voltage

feedback loop uses an internal feedback resistor divider.

Theperiodissetbytheinternaloscillator.Thedevicehasan

internalsynchronousNMOSrectifieranddoesnotrequirea

SchottkydiodeontheLXpin.Thedeviceoperatesasabuck

converter in PWM mode with a fixed frequency of 2.5MHz.

Programmable Output Voltage

The SC183C has fifteen pre-determined output voltage

values which can be individually selected by program-

ming the CTL input pins (see Table 1 — Output Voltage

Settings). Each CTL pin has an active 1 MΩ internal pull-

down resistor. The 1MΩ resistor is switched in circuit

whenever the CTL input voltage is below the input

threshold, or when the part is in undervoltage lockout. It

is recommended to tie all high CTL pins together and use

an external pull-up resistor to AVIN if there is no enable

signal or if the enable input is an open drain/collector

signal. The CTL pins may be driven by a microprocessor to

allow dynamic voltage adjustment for systems that reduce

the supply voltage when entering sleep states. Avoid all

zeros being present on the CTL pins when changing pro-

grammable output voltages as this would disable the

device.

SC183C is also capable of regulating a different (higher)

output voltage, which is not shown in the Table 1, via

an external resistor divider. There will be a typical 2µA

current flowing into the VOUT pin. The typical sche-

matic for an adjustable output voltage option from

the standard 1.0V with CTLx=[0010], is shown in Figure

1. RFB1 and RFB2 are used to adjust the desired out-

put voltage. If the RFB2 current is such that the 2µA

VOUT pin current can be ignored, then RFB1 can be

found by Equation 1. RFB2 needs to be low enough in

value for the current through the resistor chain to be

at least 20µA in order to ignore the VOUT pin current.

2

1

FB

OSTD

OSTD

OUT

FB

R

V

V

V

R

⋅

−

=

(1)

where V

CTL pins.

C

mance. The correct value of C

Equation 2.

(

)

(

)

)

5

.

0

(

]

[

5

.

0

5

.

2

]

[

1

2

−

×

−

⋅

Ω

−

×

=

OSTD

OSTD

OSTD

OUT

FB

OUT

FF

V

V

V

V

k

R

V

nF

C

(2)

To simplify the design, it is recommended to program the

desired output voltage from a standard 1.0V as shown in

Figure 1 with a proper C

programming the output voltage from other standard

voltages, R

Equation 1and 2.

Protection Features

The SC183C provides the following protection features:

Current Limit

Over-Voltage Protection

Soft-Start Operation

Thermal Shutdown

Current Limit

The internal PMOS power device in the switching stage is

protected by a current limit feature. If the inductor current

is above the PMOS current limit for 16 consecutive cycles,

the part enters foldback current limit mode and the

output current is limited to the current limit holding

•

•

•

•

Applications Information

Figure 1 — Typical Schematic for Adjusting the

Output Voltage Up from an Output Voltage of 1.0V

(CTLx=[0010])

CTL0

CTL1

PGND

AVIN

10µF

LX

VOUT

Enable

SC183C

PVIN

CTL2

AGND

CTL3

L

10k

2

1

)

1

(

FB

OUT

FB

R

V

R

×

−

=

for CTLx= 0010 (1.0V)

1µF

10